starting problems with 2004 Silverado V6 4L single cab 2WD truck

I just want to say hello to everyone and thank you to anyone who can help me out with this problem. I don't know much about working on cars or trucks (mostly small engines here) but understand the basics and this problem is beyond me. The truck as a lot of mileage on it over 200,000 go Chevy right. but it still runs good. Now for the problem, this as happened a few times now and I can't remember when it started. first the truck won't turn over no starter nothing but you can hear the fuel pump. let is sit for 3 to 4 days and then it will either turn over and still won't start or it will start right away. once it does start it runs fine doesn't die or anything. however a few days later and the whole things starts over again. the alternator and battery are new and have been checked, we think it is either the relay switch or something involving a spark plug in cylinder 5 but I'm not sure. Anyone else have this problem before or have any ideas, any help will be appreciated. plus here are the 3 trouble codes I got off the computer p0200, p0305,p0449 I know what the codes mean and got some causes and solutions from the obd website. just listing them here to see if it helps in determining the problem.